The present invention relates to an ion source in a mass spectrometric apparatus in which a sample molecule is impinged on Rydberg atom and thereby ionized negatively for mass spectrometric analysis. More particularly, the present invention is concerned with an ion source capable of producing Rydberg atoms stably and making negative ions of a sample produced by impingement on Rydberg atoms present in a stable state.
Recently, necessity has been increasing for analyzing, in a simple manner and that in high sensitivity, trace amounts of chemical substances having high molecular weights and complicated structures such as very small amounts of active substances in life, living body components, trace amounts of active substances, e.g. perfumes and offensive smell, medical substances, including vitamins, a wide variety of food additives and oil, fat antioxidants. Mass spectrometric analysis is the simplest and most economical method for analyzing these substances quickly and in high sensitivity.
In mass spectrometric analysis it is necessary to ionize a sample. Generally, a gaseous sample of a molecular compound is ionized by using electron beam (e.g. 70 V), proton, or ion beam. According to this method, the possibility of formation of cation is 10.sup.4 times that of anion, and the energy given to a sample molecule by electron impact is much larger than the amount thereof required for ionization or for bond rupture. Consequently, the ionization is followed by fragmentation and further secondary ionic molecular reactions and the complicated spectrum is obtained.
As a result, information on the parent molecule which is important particularly in analyzing the results is apt to be uncertain. In order to suppress such secondary processes upon ionization, there have been developed several methods such as techniques of field elimination ionization and high-speed atomic impingement ionization. Even these methods can be completely avoid the deterioration of a sample having a high molecular weight, extraction of information of the molecular weight and abundance of the sample molecules necessitate a complicated analysis of the decomposed ions.
For solving these problems, we have developed a method wherein a gaseous sample molecule is negatively charged by collision units, Rydberg atoms and the ions thus produced are subjected to mass spectrometric analysis. In the method used so far, wherein a gaseous sample molecule is allowed to collide with Rydberg atoms and the ions produced are sent to a mass spectrometer, the filament-grid distance is so small (see FIG. 3) that an impurity atom (e.g. tungsten atom) sputtered from the filament is vapor-deposited on the grid and discharge occurs. Repeated discharge interrupts the measurements and causes the filament burnt out. In the method in question, thermalelectrons from the filament are accelerated and collisionally excite a rare gas (e.g. xenon) into Rydberg states (Rydberg atom).
However, the density of the Rydberg atom thus produced is low because of the low current density of electrons in the vicinity of the grid. Further, the sample molecule is thermally decomposed on the wall which is heated by the hot filaments; there occurs decomposition of the molecular ions produced; the production efficiency of negative ions of the parent molecule in the sample by impact of the Rydberg atoms is deteriorated; and the spectrum obtained is very complicated, and is difficult, or as the case may be impossible, to be analyzed.
It is the object of the present invention to provide an ion source by use of Rydberg atoms impact, which is capable of overcoming the above-mentioned problems, generating Rydberg atoms stably, and producing negative ions in collision with Rydberg atoms present in a stabilized state.